1. Field of the Invention:
This invention relates in general to electrical inductive apparatus and, more particularly to a three-phase electrical transformer utilizing a cable driven mechanism.
2. Description of the Prior Art:
Tap changing has been used in various types of electrical inductive apparatus and is usually provided for power transformers to enable the output voltage of the transformer to be maintained substantially constant under varying load conditions, or to enable the output voltage to be varied. Usually the tappings are on either the primary or the secondary side of the transformer, and the tap changing mechanism is arranged so that tappings may be changed without disconnecting a load.
No-load tap changers provide a range of output voltages for transformer. Usually the transformers include a series of connections or taps from various sections of either the primary or secondary winding, whereby a different number of winding turns are employed at each tap. Conventional no-load tap changers, which operate when the transformer is deenergized, include a plurality of stationary contacts, each connected to a different tap on the winding, and a contact assembly which can be moved to engage any one of the stationary contacts, thereby altering the turns ratio between the primary and secondary windings and altering the output voltage of the transformer.
For three-phase transformers such tap changers have separate sets of stationary and movable contacts for each phase of the transformer. The movable contacts are usually connected together by a common drive means causing them to rotate together. Although satisfactory, such tap changers require detailed coupling mechanisms to alter the tap settings. The connecting members are necessarily large and rigid to ensure proper alignment of gear drive trains and to withstand large forces required to switch all three phases of the tap changer simultaneously. Such construction increases the size and weight of the transformer, because no-load tap changers are usually mounted inside the transformer enclosure. Accordingly, there is a need for a no-lead tap changer mounted inside the transformer enclosure which is smaller and more economical than the no-load tap changers of prior structures.